In recent years, astronomers, paleontologists and geologists have given us one more thing to worry about. That a large meteor or comet may strike the earth and destroy humanity or at least a good portion of it. If a collision occurred between Earth and an asteroid a few kilometers in diameter, it would release as much energy as several million nuclear weapons detonating simultaneously. This would be enough to trigger a nuclear winter. Those who survived the blast might starve to death as all plant life died. Recently we have seen a spate of science fiction movies with this theme, and it has been a staple of SF and doomsayers for a long time. How real is the threat? That is the question I intend to answer here. Note: Much of the information I gleaned from a Wikipedia article entitled Impact Event. The Wikipedia has a lot more information about possible asteroid collisions with earth and other planets in the solar system. Throughout recorded history, hundreds impact events have been reported, with some occurrences causing deaths, injuries, property damage or other significant localized consequences. An impact event in an ocean or sea could create a tsunami that would cause destruction both at sea and on land near a seashore. Small objects frequently collide with the Earth. There is an inverse relationship between the size of the object and the frequency that such objects hit the earth. Asteroids with a 1 km (0.62 miles) diameter strike the Earth every 500,000 years on average. Large collisions, with 5 km (3 miles) objects, happen approximately once every ten million years.

The last known impact of an object of 10 km (6 miles) or more in diameter was at the Cretaceous-Tertiary extinction event 65 million years ago which wiped out the dinosaurs. Asteroids with diameters of 5 to 10 meters (16 to 33 feat) enter the Earth's atmosphere approximately once per year, with as much energy as the atomic bomb dropped on Hiroshima, approximately 15 kilotons of TNT. Usually they explode in the upper atmosphere, and most or all of the solids are vaporized. Objects with diameters over 50 meters (164 feat) strike the Earth approximately once every thousand years, producing explosions comparable to the one known to have detonated above Tunguska in 1908. At least one known asteroid with a diameter of over 1 km (0.62 miles) may collide with Earth on March 16, 2880. Objects with diameters smaller than 10 meters (33 feat) are called meteoroids (or meteorites if they strike the ground). An estimated 500 meteorites reach the surface each year, but only 5 or 6 of these are recovered and made known to scientists. The most significant recorded impact on earth in recent times was the Tunguska event, which occurred in Siberia, Russia, in 1908. This incident involved an explosion that was probably caused by the air burst of an asteroid or comet 5 to 10 km (3.1 to 6.2 miles) above the Earth's surface. It caused an estimated 80 million trees over 2,150 km2 (830 square miles) to be knocked over. In my latest Morgaine novel, Morgaine and the Necromancer, this event plays a part in the plot.

Numerous science fiction stories and novels center around an impact event; possibly the best-selling was the novel Lucifer's Hammer by Larry Niven and Jerry Pournelle. In 1992 a Congressional study in the U.S. led to NASA being directed to undertake a Spaceguard Survey with the novel being named as the inspiration for the name to search for Earth-impacting asteroids. This in turn inspired Clarke's 1993 novel The Hammer of God. Jack McDevitt's 1999 novel Moonfall, in which a very large comet travelling at interstellar velocities collides with and partially destroys the Moon, fragments of which then collide with the Earth. Several disaster movies have also been made: When Worlds Collide (1951) dealt with two planets on a collision course with Earth, the smaller planet a "near miss," causing extensive damage and destruction, followed by a direct hit from the larger planet. Meteor (1979) features small asteroid fragments and a large 8 km (5 miles) wide asteroid heading for Earth. In 1998 two films were released in the United States on the subject of attempting to stop impact events: Touchstone Pictures' Armageddon, about an asteroid, and Paramount / DreamWorks' Deep Impact, about a comet. The prophesies in the Book of Revelation in the Christian bible seem to indicate that part of the devastation of the Apocalypse will be caused by objects falling down from the sky. I used this scenario in the fifth book of my Morgaine series of novels, Morgaine and Armageddon. Whether a catastrophic asteroid collision with earth will occur within our lifetime is problematical. What is certain is that sooner or later, one will strike the earth. Will we be ready to either divert it or deal with the consequences?